This invention relates to a magentic recording medium, and more specifically to a magnetic recording medium which exhibits good self stability and running performance in high temperature and humidity environments in addition to improved magnetic powder dispersion and surface properties of the magnetic coating film.
In general, a magnetic recording medium such as magnetic tape, etc. is made by coating a base of polyester film or the like with a magnetic coating material prepared by dispersing a magnetic powder, for example, finely divided .gamma.-Fe.sub.2 O.sub.3, in a high-molecular resinous binder. The properties of the magnetic coating film have much to do not only with the running performance and other physical properties of the tape but also with the magnetic powder dispersion and magnetic properties of the medium. The magnetic film properties, in turn, are largely dependent on the composition of, and the additives used in, the resinous binder. In view of these, many different resinous binder ingredients have hitherto been proposed. For instance, vinyl chloride-vinyl acetate-vinyl alcohol copolymers have been widely used as a binder resin ingredient for magnetic tapes. This type of copolymers is characterized by relatively low cost, ready solubility in solvents, good compatibility with other resins, and affinity for magnetic particles on account of the hydroxyl group in the molecule which facilitates the dispersion of the particles. Additional advantages of the copolymers are excellent magnetic properties they contribute, including high degrees of orientation and maximum residual magnetic flux density.
The recent trend toward the use of finer magnetic particles has, however, made these resins not always satisfactory. Investigations have therefore been made about increasing the proportion of hydroxyl groups in the copolymers of the foregoing type. As a result, it has already been found that the dispersibility of magnetic particles is properly improved by introducing the OH groups until the OH/CH absorption ratio according to the infrared absorption spectrum reaches 0.7 or more. However, the copolymers with increased hydroxyl group contents have such high glass transition temperatures of 70.degree. C. or upward that they are difficult to process on the surface and hence the resulting magnetic layer after calendering can hardly be improved in surface quality.
In this connection we previously found that both the surface quality of the coating film and the glossiness of the calendered surface can be improved, without impairing the dispersibility of the magnetic powder, by mixing the vinyl chloride-vinyl acetate-vinyl alcohol copolymer as the binder ingredient with a resinous ingredient with a glass transition point lowered to 65.degree. C. or downward (Japanese Patent Application No. 94016/1981). This time the method showed a drawback of inadequate shelf stability and running performance under high-temperature high-humidity conditions due to the lowered glass transition point. In an effort to eliminate this drawback, we then proposed to employ a copolymer composed of vinyl chloride, a vinyl alkylcarboxylate, another monomer copolymerizable with vinyl chloride, and a saponified vinyl alkylcarboxylate and having an OH/CH absorption ratio according to the infrared absorption spectrum of 0.2 to 0.7. The copolymer, with the proportion of the saponified ingredient kept rather small, is prevented from undergoing a boost in the glass transition point. Partial loss of the dispersibility of the magnetic powder due to the decrease in proportion of the saponified ingredient is made up for with the action of the monomer copolymerizable with vinyl chloride. Thus, improvements are achieved in orientation, maximum residual magnetic flux density, and other characteristics (Japanese Pat. App. No. 182254/1981, etc.).
Nevertheless, with the more recent tendency toward the adoption of even finer magnetic particles, the singular use of such a copolymer as a binder resin poses problems yet to be solved including unsatisfactory powder dispersibility, inadaquate bond strength, and questionable storability and running performance of the resulting magnetic tapes in high-temperature high-humidity environments.
In order to solve these problems, we tried to adopt a resinous binder comprising (i) a cpolymer composed of vinyl chloride, a vinyl alkylcarboxylate, another monomer copolymerizable with vinyl chloride, and a saponified vinyl alkylcarboxylate as its constituents and having an OH/CH absorption ratio according to the infrared absorption spectrum of at least 0.2, (ii) a rubbery binder ingredient, and (iii) a polyisocyanate curing agent. The resin binder was found superior to the binder consisting of the above copolymer alone in imparting the coating film with good dispersibility, ease of calendering, and in improving the shelf stability and running performance of the magnetic medium in hot and humid environments. Still, it has been observed that even such a binder of ternary resin system cannot confer fully satisfactory dispersibility on the ultrafine magnetic particles of the present day, and that the rubbery binder ingredient added to improve the physical properties, for example, the bond strength and running quality, of the coating film rather reduces the dispersibility of the magnetic particles.
With the foregoing in view, the present invention aims at providing a magnetic recording medium with notably improved dispersion of ultrafine magnetic particles in the magnetic layer.